For forming a color image, a method has been known by which a latent image corresponding to a color is formed on one static image carrying member, usually an electrophotographic photo receptor (sometimes simply referred to as a photo receptor), and developed and transferred, and such the process is repeated for each of colors to form the color image.
In such the color image forming method, the latent image carrying member is uniformly charged and given the first exposure; this formed latent image is developed to form the first image. Then the second uniform charge is given to the latent image carrying member without the transferring of the first developed image, and the second latent image is formed by the second exposure and developed by the second development to form the second image on the latent image carrying member. In the case of full color printing, such the processes are performed as to each of colors of yellow, magenta, cyan and black, the colors are referred each to as the unit color, to form a full color image constituted by the four colors on the latent image carrying member. The toner image is collectively transferred onto an image supporting material such as a paper sheet and fixed to form the image.
As another method for forming a full color image, a method is known in which latent image carriers are prepared for each of the colors and the images formed on each of the latent image carriers are repeatedly transferred onto the same area of the image supporting member to form the full color image.
This method has an advantage that the method corresponds to a high speed image formation since the latent image forming process and the developing and transferring processes are prepared for each of the color units and the speed for the monochromatic image is the same as the speed for forming the full color image. In this method, it is necessary to stabilize the developing amount for controlling the color balance since the color images of each color units are separately formed on the latent image forming members different from each other. Moreover, a problem is raised on the stability of the final image quality when the adhesiveness of the toner of the color units are different from each other since the toner images each formed on each of the latent image carriers are transferred to the image support and fixed to form the image. Furthermore, the difference of the position between each of the color units tends to be occurred on the transfer and the problem of the disagreement of the color image position is caused. Consequently, it is difficult to stably form the images for a long period.
Besides, an usual toner prepared by the crashing method causes a problem of lowering the color reproducibility of the color image since the material dispersed in the toner is not uniformly distributed at the crashed surface of the toner and the surface property of the each of the toner particle is difficultly to be the same, consequently, the stabilization of the adhering amount of toner and the unifying the adhesiveness of each of the color units can be difficultly realized.
Therefore, the toner prepared by the polymerization method so called as the polymerized toner is recently noticed. Among the polymerized toners, a suspension polymerized toner is expected to have a high uniformity of the toner particles since the toner particle produced by such the method has a sphere shape and a uniform surface property. However, the sphere-shaped toner tends to cause lowering the transferring ability and the shedding of image on fixing since such the particle shows excessive adhesiveness to the static latent image carrying member and the image support.
Namely, the stable formation of the image for a long period and the stability depending on the environmental condition cannot be obtained in the process of the successively transferring the images each formed on the latent image carrying members by the method for forming the full color image, so called as tandem method, using the static latent image carrying members for each of the color units.
The method by which plural toner images are formed on the photoreceptor and collectively transferred onto the image support such as a paper sheet using no intermediate transferring member has an advantage such as that the apparatus can be made compact. However, problems are raised on the method such as that the roughing of the image is occurred on the transfer and the mixing of the different color toners is occurred on the image formation. Therefore, it is difficult to obtain sufficient images for a long period.